Conventionally, a flow rate of fluid flowing in a channel having various shapes such as an annular channel can be measured by using an electromagnetic flowmeter. There is known, as such an electromagnetic flowmeter, a technique disclosed in Patent Document 1. In the technique disclosed in Patent Document 1, a magnetic field generator is set such that both magnetic poles are arranged at one side of an outer wall of a pipeline.
More specifically, the technique disclosed in Patent document 1 includes an excitation device for forming a magnetic field perpendicular to a pipeline outer wall and a pair of electrodes for measuring electromotive force generated when conductive fluid crosses the magnetic field. The electrodes and both magnetic poles of the excitation device are arranged at one side of the pipeline outer wall.